A reinforcing structure in which a plurality of carcass layers is laid between a pair of bead portions in order to maintain high internal pressure is used in pneumatic tires. For example, a pneumatic tire has been proposed having a so-called 2-1F ply lock structure in which three carcass layers are laid between a pair of bead portions, and both ends of the two inner carcass layers are folded back from a tire inner side to a tire outer side around bead cores, while both ends of the outermost carcass layer are disposed to the outer side of the folded back portions of the inner carcass layers in a tire width direction (see, for example, Japanese Unexamined Patent Application Publication No. H11-321217).
FIG. 8 schematically illustrates a conventional pneumatic tire having a 2-1F ply lock structure. As illustrated in FIG. 8, both ends of inner carcass layers 41, 42 are folded back from the tire inner side to the tire outer side around a bead core 5, and both ends of an outer carcass layer 43 are disposed to the outer side of the folded back portions of the inner carcass layers 41, 42. In a pneumatic tire having such a 2-1F ply lock structure, the presence of the three carcass layers 41, 42, 43 in the side wall portions allows for excellent steering stability.
However, pneumatic tires are frequently subjected to severe usage conditions, such as high load states caused by vehicle overloading or high internal pressure states intended to ensure load capacity; thus, if the ends of multiple carcass layers are disposed at locations of the bead portions or side wall portions where flexing tends to occur, separation failure originating at these ends easily occurs. In addition, the use of three carcass layers also increases the tire weight, leading to a problematic increase in the rolling resistance of the tire.
The problems described above can be overcome by reducing the number of carcass layers, but this will reduce the rigidity of the tire as a whole, potentially reducing steering stability.